"Work it" usually means, "strut your stuff," show off your assets, make some physical attribute (figure, walk, style of dress, etc.) to "work" for you to make you attractive.
While not proven or confirmed, the expression may have originated among "working girls," i.e. prostitutes/street walkers, who would make a sultry walk etc. "work" for them to attract business. If it did have that connotation, it has been lost with time, and now usually is applied more innocently.